Add iodine. For a novice, pick up a solution called "tech I" or really any iodine solution you can dig up I supose, but be careful about OD'ing.
I have seen xenia thrive in water with 60ppm nitrate, so I dont belive thats part of the problem at all. If you did a water change and the xenia improved a bit, I belive it was because the new salt mix contained a bit of Iodine in it which they needed.
Gotta act fast on this, you only have about a 1-2 day window from the time it acts funny until it melts down like it never existed.
I hope this helps you.